The use of implants to affix tissue grafts to bone is well known in the orthopedic arts. Common procedures in which such implants are used include, for example, the repair of rotator cuff tears, the repair of torn ligaments in the knee, among others. When using a first type of anchoring implant, the implant (commonly referred to as an “anchor” or “suture anchor”) is placed in a socket prepared in the surface of a target bone and the target tissue (also referred to as a “graft”) is secured to the bone by sutures passing through the anchor. A second type of anchoring implant commonly referred to as an “interference screw” secures tissue to bone by trapping it between the implant and the wall of a socket formed in the bone. In either context, the socket is generally formed by drilling or punching at a predetermined location. The graft, generally a tendon or ligament, is inserted into the socket and the interference screw is threaded into the socket thereby securing tissue trapped between the implant and the wall of the socket. Implants of both types may be formed from metallic materials, polymers, or bioabsorbable materials. Metallic anchors have a high pull-out strength, but remain in the body of the patient indefinitely. On the other hand, bioabsorbable materials degrade in the body, but the reduced strength of the materials necessitate designs having thicker cross-sections which, in turn, limits the sizes and configurations in which these anchors can be produced.
Janko, et al. in U.S. Pat. Nos. 8,591,672 and 8,246,762 describe “ . . . medical devices comprising high-strength alloys which degrade over time in the body of a human or animal, at controlled degradation rates, without generating emboli.” See Abstract of Janko '672. The Janko invention is intended to provide “implantable medical devices comprising a biodegradable alloy that dissolves gradually from its exterior surface” wherein, in certain embodiments, “the rate of dissolution from the exterior surface of the alloy is substantially uniform across smooth portions of the exterior surface.” See Janko '672, col. 2, lines 26-31. Janko recites a number of devices that may advantageously be formed from these materials. Quoting Janko '672, at col. 3, lines 45-52: “In certain embodiments, the implantable medical device is a high tensile bone anchor (e.g., for the repair of separated bone segments). In other embodiments, the implantable medical device is a high tensile bone screw (e.g., for fastening fractured bone segments). In other embodiments, the implantable medical device is a high strength bone immobilization device (e.g., for large bones). In other embodiments, the implantable medical device is a staple for fastening tissue.” Further quoting Janko '672, from col. 15, lines 5-17: “A substantially cylindrical device, which would lose surface area linearly with the loss of diameter as the device degrades, could have a concentric hole drilled through the center of the device. The resulting cavity would cause a compensating increase in surface area as alloy was dissolved from the luminal surface of the device. As a result, the change in surface area as the device degrades over time—and thus the change in rate of degradation—would be minimized or eliminated. A similar strategy of creating a luminal space (e.g., a luminal space that has a shape similar to the outer surface of the device) could be implemented with essentially any type of medical device.”
The techniques for increasing the ratio of surface area to volume taught by Janko are readily applied to fasteners of cylindrical geometry, such as screws. However, contrary to Janko's suggestion that “a luminal space that has a shape similar to the outer surface of the device) could be implemented with essentially any type of medical device”, it is in fact not possible to produce a luminal space in conventional metal suture anchors currently available for affixing soft tissue to bone, examples of which include the Revo Suture Anchor by Conmed, Inc. (Utica, N.Y.) and the Ti-Screw Suture Anchors by Zimmer Biomet (Warsaw, Ind.). FIGS. 1A and 1B depict a prior art metal anchor 100 of typical construction having a threaded distal portion and a proximal drive portion with a proximally positioned eyelet through which suture is passed. Skilled artisans will agree that it is not possible to produce the anchor of FIGS. 1A and 1B with a central lumen. If a biodegradable alloy were to be substituted for the standard alloy from which anchor 100 is currently constructed, the anchor would dissolve in the manner taught by Janko, that is, from the external surfaces inward. Absorption of the anchor in this manner allows portions of the anchor to remain in the body well past the time at which the anchor has the ability to function effectively to maintain tissue in contact with bone, and may allow first portions of the anchor to become detached from a second portion of the anchor that remains secured in the bone so as to create loose bodies in the region, a negative effect that can have very serious consequences.
Accordingly there remains a pressing need in the art for a metallic bioresorbable tissue anchoring implant configured such that all portions of the implant degrade over a short predetermined period of time while maintaining it tissue anchoring capability and preventing the generation of loose bodies therefrom.